Butt wink

The "Butt Wink" is not always tightness. What people seem to miss is the possible causes of this. One possibility for a pelvic tilt is bone structure, this isn’t always caused by a tightness. If you are training and this is an issue, it could be caused by limited dorsi flexion, a long femur in comparison to tibia and fibula, hip socket depth or the femur neck length and angle. 

often people are encouraged to go “ass to grass”, but if this isn’t structurally viable, maybe this person should not go as deep. Sometimes a squat may not be the best exercise for a person with these issues, or the range needs to be limited to their safe ranges rather than the choreography trainer have pictured for a perfect person. Just something to think about next time you see somebody “doing half reps” and if your trainer tries to force you in uncomfortable positions.